Method for producing a zinc-magnesium-galvannealed hot-dip coating and flat steel product provided with such a coating

ABSTRACT

A process may be utilized to produce a galvannealed Zn—Mg hot dip coating that provides improved corrosion protection on a steel substrate, and to a flat steel product having such characteristics. To this end, a steel substrate may be subjected to hot dip coating in an Fe-saturated melt bath heated to 350-650° C., wherein the melt bath comprises in percent by weight 0.1-0.16% Al, 0.1-0.6% Mg, and also &lt;2% Si, &lt;1% each of Ni or Cu, &lt;0.3% Co, &lt;0.0001% Mn, &lt;0.5% Sr, &lt;0.1% each of Pb, B, Bi or Cd, &lt;0.2% each of REM, Ti or Cr, or &lt;0.5% Sn, with the balance being zinc and unavoidable impurities. Further, a galvannealing treatment may be conducted in which the steel substrate is kept at 450-800° C. for 10-25 seconds to produce a galvannealed Zn—Mg hot dip coating on the steel substrate. The coating may comprise Zn, unavoidable impurities, and in percent by weight 0.10-0.5% Al, 5.0-15.0% Fe, 0.10-0.8% Mg and &lt;2 % Si, &lt;1% each of Ni, Cu, &lt;0.3% Co, &lt;0.0001% Mn, &lt;0.5% Sr, &lt;0.1% each of B, Bi, Cd, Pb, &lt;0.2% each of Cr, Ti, REM, &lt;0.01% Sn.”

The invention relates to a process for producing a galvannealed zinc-magnesium hot dip coating on a steel substrate, wherein the steel substrate is typically a flat steel product. A “flat steel product” refers to rolled steel products such as strips, sheets, and (pre-cut) blanks obtained therefrom.

The invention further relates to a flat steel product coated with a galvannealed zinc-magnesium hot dip coating.

A process and a flat steel product of a corresponding nature are known from WO 2009/059950 A2. In the known process, the flat steel product is annealed at an annealing temperature of 500-900° C., then cooled down to a bath entry temperature in the range of 360−710° C. and then guided through a Zn—Mg melt bath which has been heated to a melt bath temperature of 350-650° C. and may comprise, as well as zinc and unavoidable impurities (in % by weight), 4-8% Mg and 0.5-1.8% Al, and optionally one or more of the following elements with a content below the upper limit specified for each of these elements: Si: <2%, Pb: <0.1%, Ti: <0.2%, Ni: <1%, Cu: <1%, Co: <0.3%, Mn: <0.5%, Cr: <0.2%, Sr: <0.5%, Fe: <3%, B: <0.1%, Bi: <0.1%, Cd: <0.1%. In the flat steel product that exits from the melt bath, the layer thickness of the metallic coating is adjusted by removing excess Zn—Mg melt. The flat steel product thus obtained has a Zn—Mg—Al coating which, as well as zinc and unavoidable impurities, comprises (in % by weight) Mg: 4-8% and Al: 0.5-1.8%, and optionally one or more of the following elements with a content below the upper limit specified for each of these elements: Si: <2%, Pb: <0.1%, Ti: <0.2%, Ni: <1%, Cu: <1%, Co: <0.3%, Mn: <0.5%, Cr: <0.2%, Sr: <0.5%, Fe: <3%, B: <0.1%, Bi: <0.1%, Cd: <0.1%, REM <0.2%, Sn<0.5%. The flat steel product thus coated has excellent protection against corrosion and good suitability for welding.

Practical experience shows that, in the case of a galvannealed hot dip coating system, owing to the elevated Fe content in the coating resulting from the galvannealing treatment, there is premature occurrence of iron corrosion products that impair the anticorrosive action.

Against the background of the prior art, it was an object of the invention to name a process that allows the coating of steel substrates with a galvannealed hot dip coating which offers improved corrosion protection. A flat steel product correspondingly provided with an optimized galvannealed coating was likewise to be named.

In relation to the process, this object has been achieved by the invention in that, in a process of this kind, the steps specified in claim 1 are performed.

A flat steel product that achieves the aforementioned object in accordance with the invention is specified in claim 10.

Advantageous configurations of the invention are specified in the dependent claims and are elucidated individually hereinafter, as is the general concept of the invention.

The invention provides a process for producing steel products, especially flat steel products, that have been provided with a zinc-based hot dip coating and have improved anticorrosion properties. For this purpose, magnesium is introduced into the coating. To maintain the processing properties of the galvannealed hot dip coating, the maximum magnesium content that can be included in the alloy is limited here in order to still assure the alloy-forming characteristics of the galvannealed hot dip coating.

The steps that are typically implemented in a process for hot dip coating of a flat steel product with a Zn—Mg—Al coating, which are not mentioned individually here, are elucidated in WO 2009/059950 A2 which has already been mentioned above, the contents of which are hereby incorporated by reference into the present application for completion of the disclosure.

According to the invention, in a departure from the prior art known from WO 2009/059950 A2, in accordance with the invention, the composition of the melt bath is chosen such that, in the subsequent galvannealing treatment, a galvannealing coating with optimal use and anticorrosion properties is established. The aim of the procedure of the invention is the production of a hot dip coating with a Fe content typical of a galvannealed coating of 5-15% by weight, especially 7-15% by weight.

Accordingly, in a process of the invention for producing a galvannealed zinc-magnesium hot dip coating on a steel substrate, at least the following steps are implemented:

-   a) providing a steel substrate, -   b) hot dip coating of the steel substrate in an Fe-saturated melt     bath heated to a bath temperature of 350-650° C., consisting of (in     % by weight) 0.1-0.16% Al and 0.1-0.6% Mg, and in each case     optionally <2% Si, <0.1% Pb, <0.2% Ti, <1% Ni, <1% Cu, <0.3% Co,     <0.0001% Mn, <0.2% Cr, <0.5% Sr, <0.1% B, <0.1% Bi, <0.1% Cd, <0.2%     REM or <0.5% Sn, the balance being zinc and unavoidable impurities     from the production, and -   c) performing a galvannealing treatment in which the hot dip-coated     steel substrate is kept at a galvannealing temperature of     450-800° C. over a galvannealing time of 10-25 s in order to produce     a galvannealed zinc-magnesium hot dip coating on the steel     substrate, comprising zinc and unavoidable impurities (in % by     weight) 0.10-0.5% Al, 5.0-15.0% Fe, 0.10-0.8% Mg and optionally one     or more of the following elements with the following proviso     regarding their contents: Si: <2%, Pb: <0.1%, Ti: <0.2%, Ni: <1%,     Cu: <1%, Co: <0.3%, Mn: <0.0001%, Cr: <0.2%, Sr: <0.5%, B: <0.1%,     Bi: <0.1%, Cd: <0.1%, REM: <0.2%, Sn <0.01%.

The selection of the parameters in the process of the invention is justified as follows:

-   -   Al content in the melt bath:     -   The Al content in the melt bath is 0.10-0.16% by weight, in         order to assure good adhesion of the coating produced in         accordance with the invention on the steel substrate. The         addition of aluminum to the melt bath brings about the formation         of an Fe2Al5 inhibition layer between the base material and the         coating. This Fe2Al5 inhibition layer firstly assures the         bonding of the coating to the steel substrate. Secondly, the         Fe2Al5 inhibition layer inhibits the diffusion of the iron from         the base material into the coating. The Fe2Al5 layer must not be         too indistinct because the bonding between substrate and coating         is otherwise impaired. However, it must not be too pronounced         either because the diffusion of the iron into the coating is         otherwise hindered too significantly. This mechanism is         applicable both to the limitation of the Al content in the melt         bath and to the limitation of the Al content in the coating.         Since Al is accumulated in the coating to a greater degree than         it is present in the melt bath, different upper limits are         applicable to bath and coating. The Al contents of the melt bath         that are envisaged in accordance with the invention are such as         to result in a layer structure which is optimal for the adhesion         of the coating. If the aluminum content in the melt bath exceeds         the upper limit of 0.16% by weight, a covering Fe₂Al₅ inhibition         layer will form between the base material and the coating, which         is so thick that no proper galvannealed coating can be formed.         If the aluminum content is less than 0.10% by weight, adhesion         of the coating is inadequate. Harmful effects of the Al content         on the formation of an optimal galvannealed coating can be         reliably avoided in that the Al content of the melt bath is         limited to not more than 0.15% by weight, and the positive         effect of the presence of Al in the melt bath occurs         particularly reliably when the Al content is more than 0.1% by         weight, i.e. for the purposes of the person skilled in the art         there is distinctly more than 0.1% by weight of Al present in         the melt bath.     -   Fe content in the melt bath:     -   The melt bath should be saturated in Fe in order that the         driving force for diffusion of iron out of the steel base         material into the melt bath is hindered. The provision that the         melt bath is to be saturated in Fe corresponds in practice, with         the other specifications that are applicable here, to an Fe         content in the melt bath of 0.01-0.5% by weight. If the iron         content in the melt bath is less than 0.01%, iron can diffuse         out of the steel base material into the melt bath. This damages         the structure of the base material. The upper limit in the Fe         content in the melt bath is determined by the solubility limit         of the iron in the melt bath. If this limit is exceeded, there         can be increased formation of slag resulting from precipitating         iron phases.     -   Mg content in the melt bath:     -   The Mg content in the melt bath is 0.10-0.6% by weight. The         upper limit of 0.6% by weight is applicable in order to achieve         alloy formation in the coating at technically and economically         favorable galvannealing temperatures of up to 800° C. It is         found to be particularly practical in this aspect when the Mg         content of the melt bath is restricted to up to 0.4% by weight.         In the case of contents of less than 0.1% by weight, however,         the improvement in corrosion protection which is the aim of the         addition of Mg to the melt bath is achieved only inadequately.         It can therefore be advantageous to add distinctly more than         0.1% by weight to the melt bath for the purposes of the person         skilled in the art.     -   Other elements present in the melt bath:     -   The balance not accounted for by the contents of Al, Fe and Mg         in the melt bath provided in accordance with the invention         consists of zinc, where it is optionally possible in each case         for the melt bath to include (in % by weight), less than 2% Si,         less than 0.1% Pb, less than 0.2% Ti, less than 1% Ni, less than         1% Cu, less than 0.3% Co, less than 0.0001% Mn, less than 0.2%         Cr, less than 0.5% Sr, less than 0.1% B, less than 0.1% Bi, less         than 0.1% Cd, less than 0.2% REM and less than 0.5% Sn. These         optionally present elements are present in the form of         impurities from the preparation that are unavoidable but         inactive with regard to the behavior of the melt bath and the         coating to be produced therefrom, or may be deliberately added         to establish particular properties of the coating. What is         noteworthy here is that the Mn content of the melt bath is         always restricted to a maximum value at which Mn is not present         in the melt bath for technical purposes and as such displays no         effect at all.     -   Melt bath temperature TB:     -   The melt bath temperature TB should be 350-650° C. At         temperatures below 350° C., the melt bath begins to solidify and         the steel substrate can no longer be coated. At temperatures         greater than 650° C., there is increased evaporation of zinc.         This is hazardous to health and leads to soiling of the coating         plants. Melt bath temperatures of at least 430° C. have been         found to be particularly useful in practice. Over and above a         melt bath temperature of at least 430° C., it is possible to         guarantee an optimally fluid melt bath with which coatings with         optimized runoff properties and likewise optimized surface         quality and homogeneity of the coating thickness can be achieved         on the respective steel substrate. By limiting the melt bath         temperature to 490° C., it is additionally possible to reduce         the zinc loss resulting from evaporation, the endangerment of         the environment and soiling to a minimum. Optimal melt bath         temperatures of the Zn—Mg—Al—Fe melt bath envisaged in         accordance with the invention are accordingly in the range of         430-490° C.     -   Galvannealing temperature TG:     -   The galvannealing temperature TG in the process of the invention         is to be 450-800° C. The galvannealing temperature has to be at         least 450° C. in order to activate the diffusion of iron out of         the steel substrate into the hot dip coating. In order to be         able to utilize this effect in an operationally reliable manner,         the galvannealing temperature TG can be adjusted to at least         540° C. At temperatures above 800° C., by contrast, there is the         risk that the zinc layer will evaporate off to an enhanced         degree. This unwanted effect can be avoided in a particularly         reliable manner in that the galvannealing temperature TG is         restricted to a maximum of 720° C. Experiments have shown that,         with the Mg contents that have been envisaged in accordance with         the invention in the melt bath, the temperature range of         540-720° C. is optimal when the galvannealing times are chosen         in the manner elucidated hereinafter. It has been found that         particularly reliably zeta phase-free coatings can be produced         with galvannealing temperatures of 540-720° C.     -   Galvannealing Time:     -   The galvannealing time, i.e. the time over which the flat steel         product provided with the hot dip coating is kept at the         galvannealing temperature, is 10-25 s. The galvannealing time         has to be at least 10 s in order to achieve the minimum content         of Fe envisaged in accordance with the invention in the coating         and the associated formation of the characteristic galvannealing         coating phases. In order to assure this in an operationally         reliable manner, a galvannealing time of at least 12 s can be         provided. At the same time, in accordance with the invention,         the galvannealing time is not to exceed 25 s in order to avoid         overalloying of the coating, i.e. a rise in the Fe contents of         the coating to values of more than 15% by weight. The         galvannealing time is variable in principle within the limits         defined in accordance with the invention and can be influenced         by the rate with which the steel substrate coated in each case         is passed through the galvannealing treatment.     -   An overalloyed coating would not result in a sufficient         anticorrosive effect. In order to particularly reliably avoid         overalloying, the galvannealing time can be limited to not more         than 24 s. Coatings having optimal properties can be produced         unerringly in an economically viable manner when the         galvannealing time is 12-24 s and the galvannealing temperature         is 540-720° C.     -   Layer thickness of the coating obtained:     -   The layer thickness of the metallic coating produced in         accordance with the invention is typically 3-20 μm. Layer         thicknesses within this range, with optimal use properties and         forming characteristics of the steel substrate coated in         accordance with the invention, achieve corrosion protection         which is likewise optimal.     -   Degree of alloy formation in the coating obtained:     -   It is basically the case that the degree to which an alloy is         formed is defined via the Fe content in the coating. The coating         is considered to be alloyed when the iron content in the coating         is 9% by weight or more, whereas it is considered to be         “overalloyed” when the Fe content in the coating is greater than         15% by weight.     -   Al content in the coating:     -   The Al content in the coating is 0.10-0.5% by weight. Al         contents of this kind are necessary to ensure bonding of the         coating, and are established as a result of the Al content of         the melt bath envisaged in accordance with the invention. The         Fe₂Al₅ inhibition layer formed in the coating ensures good         bonding of the coating and improves the forming characteristics.         The reason for the higher content in the coating compared to the         melt bath likewise lies in the Fe₂Al₅ inhibition layer, since Al         from the bath is increasingly accumulated in reaction with the         Fe from the steel here. If the aluminum content in the coating         is less than 0.10% by weight, what is formed is an inadequate         Fe₂Al₅ inhibition layer that is required for sufficient         adhesion. If the aluminum content in the coating is greater than         0.5% by weight, what is formed is an Fe₂Al₅ inhibition layer         which is so thick that the diffusion of iron into the coating is         so significantly inhibited that no proper galvannealed coating         can be formed.     -   Fe content in the coating obtained:     -   The Fe content in the coating is 5.0-15.0% by weight. In order         to count as a galvannealed coating, the iron content in the         coating in customary production has to be at least 5.0% by         weight. As elucidated above, alloy formation can be assumed when         the iron content of the coating is at least 9% by weight.         Advantageously, the Fe content in a flat steel product coated         with a hot dip coating in accordance with the invention is         9.0-13.0% by weight.     -   Mg content in the coating obtained:     -   The Mg content in the coating is 0.10-0.8% by weight. At this Mg         content, improved corrosion protection compared to a standard         galvannealed coating without Mg is achieved. If the Mg content         in the coating is less than 0.10% by weight, no improvement in         corrosion protection is detectable. If the Mg content in the         coating is more than 0.8%, the galvannealing temperatures         necessary have to be increased to such an extent that         performance in conventionally available production plants can no         longer be implemented viably in technical and economic terms. As         a result of the formation of Zn—Mg phases, Mg is incorporated         into the coating with higher proportions than it is dissolved in         the melt bath. This effect corresponds to the processes already         elucidated above in connection with the Al content of a coating         created in accordance with the invention.     -   Other elements present in the coating obtained:     -   The main constituent of the coating produced in accordance with         the invention is zinc. In addition, as well as the main alloy         elements Al, Mg and Fe that have already been mentioned, the         coating may include the unavoidable impurities from the         preparation and optionally one or more of the following elements         each with a content below the upper limit specified for each of         these elements (in % by weight): Si: <2%, Pb: <0.1%, Ti: <0.2%,         Ni: <1%, Cu: <1%, Co: <0.3%, Mn: <0.0001%, Cr: <0.2%, Sr: <0.5%,         B: <0.1%, Bi: <0.1%, Cd: <0.1%, REM <0.2%, Sn<0.01%. In         accordance with the comments that have already been made above         with regard to the other elements present in the melt bath, the         elements in question may be present in the manner of impurities         in contents that are ineffective for alloying purposes or may be         deliberately added at elevated contents for establishment of         particular properties of the coating. The contents of Mn should         in each case be kept so low that they display no effect. Ca, Be         and Li are also permitted merely as impurities in a coating of         the invention, where the contents of Ca, Be and Li should be         limited to less than 0.0001% by weight in each case.

In principle, the steel substrate processed in accordance with the invention may be any steel component, for example a steel profile or the like. However, the invention is of particularly good suitability for the processing of flat steel products as steel substrate since flat steel products of this kind can be processed in the manner of the invention with high economic viability in plants established in practice. More particularly, plants suitable for this purpose are those which are passed by the respective flat steel product in a manner known in principle in a continuous run.

The application of the invention is not limited to steel products that are produced from a particular steel type, but is suitable for coating of all steel strips and sheets on which particular demands are made with regard to corrosion protection. For the treatment by the process of the invention, therefore, suitable steel products are all of those which consist of steels which can be coated at all with a galvannealed Zn—Mg—Al coating of the type to be produced in accordance with the invention. This especially include IF steels, especially soft or higher-strength IF steels, bake-hardening steels, microalloyed steels and multiphase steels. A selection of alloy specifications which is illustrative of these steels is summarized in table 6.

In accordance with the above elucidations, a flat steel product of the invention has a galvannealed zinc-magnesium hot dip coating consisting of (in % by weight)

-   -   Al: 0.10-0.5%     -   Fe: 5.0-15.0%     -   Mg: 0.10-0.8%     -   optionally of one or more of the following elements with the         following proviso regarding their contents:         -   Si: <2%,         -   Pb: <0.1%,         -   Ti: <0.2%,         -   Ni: <1%,         -   Cu: <1%,         -   Co: <0.3%,         -   Mn: <0.0001%,         -   Cr: <0.2%,         -   Sr: <0.5%,         -   B: <0.1%,         -   Bi: <0.1%,         -   Cd: <0.1%,         -   REM: <0.2%,         -   Sn: <0.01%,     -   the balance being zinc and unavoidable other impurities.

A particular product feature that should be emphasized is that it is possible to produce coatings created in the manner of the invention in the above-defined mode of the invention that are free of zeta phases and therefore show relatively low abrasion in forming operations.

The invention is elucidated in detail hereinafter with reference to working examples. The figures show:

FIG. 1 a schematic diagram of an annealing cycle performed on flat steel product samples of the invention;

FIG. 2 a diagram showing, by way of example, the shift in the current density potential curve of flat steel product samples consisting of an IF steel of variant 2 specified in table 1, these having been provided in the manner of the invention with galvannealed coatings with a rising Mg content.

Cold-rolled flat steel product samples consisting of IF steels of variants 1 and 2 specified in table 1 have been provided.

The flat steel product samples of corresponding composition have been pretreated in a manner which is known per se and has been described in detail in WO 2009/059950 A2 for the hot dip coating.

As shown schematically in FIG. 1, the samples, after degreasing, in a continuous run, have first undergone a heat treatment in which they have been subjected to recrystallization annealing, in order then to run through a melt bath at a particular intake temperature TE. Different melt baths have been used here in different experiments, the compositions of which are specified in table 2. The melt bath temperature in each case was 460° C.

On exit from the melt bath, the thickness of the hot dip coating now present on the particular sample has been adjusted to 7 μm in each case in a manner which is likewise known per se (WO 2009/059950 A2).

Subsequently, the samples have undergone a galvannealing treatment in which they have been kept at a galvannealing temperature TG over a galvannealing time tG.

After the cooling, the Al, Mg and Fe contents of the galvannealing coatings present on the samples after the galvannealing treatment have been determined. The results of this analysis and the galvannealing parameters “galvannealing time tG” and “galvannealing temperature TG” are reported in table 3.

It is found that, in the context of the invention, in the case of selection of a suitable galvannealing temperature TG, depending on the Mg content, it is possible to produce galvannealed hot dip coatings having inventive Fe contents of 8-15% by weight combined with Mg content of 0.1-0.5% by weight.

The analysis for examining the metallic coatings was conducted by means of ICP-OES according to DIN EN ISO 11885.

In further experiments, the effect of the magnesium content of the melt bath used in each case on the alloy formation and anticorrosion properties was examined in the samples consisting of IF steel variant 1 and IF steel variant 2. The contents Al_(bath) and Mg_(bath) bath of Al and Mg in the melt bath used in each case, the galvannealing temperature TG and the presence of zeta phases (ζ) are reported for the samples consisting of IF steel variant 1 in table 4 and for the samples consisting of IF steel variant 2 in table 5. In addition, tables 4 and 5 also state in each case whether an alloy has formed in the coating and whether there has been an improvement in the anticorrosion characteristics.

The presence of zeta phases (ζ) was examined by means of XRD (x-ray diffractometry) measurements.

TABLE 1 Steel C Si Mn P S Al N Ti IF steel variant 1 0.0016 0.019 0.14 0.008 0.007 0.029 0.0025 0.075 IF steel variant 2 0.0024 0.076 0.13 0.010 0.008 0.022 0.0025 0.068 Content figures in % by weight, the balance being iron and unavoidable impurities

TABLE 2 Al Fe Mg Inventive ZF 0.129 0.031 — NO ZF-Mg0.057% 0.144 0.028 0.057 NO ZF-Mg0.100% 0.125 0.029 0.100 YES ZF-Mg0.189% 0.140 0.029 0.189 YES ZF-Mg0.449% 0.136 0.019 0.449 YES Content figures in % by weight, the balance being zinc and unavoidable impurities

TABLE 3 Sample Coating no. Steel tG [s] TG [° C.] Melt bath Al Fe Mg Inventive? 1 IF steel variant 1 15 540 ZF (reference) 0.22 9.09 <0.01 NO 2 IF steel variant 2 15 540 ZF (reference) 0.26 11.58 <0.01 NO 3 IF steel variant 1 10 540 ZF-Mg0.057% 0.18 7.88 0.04 NO 4 IF steel variant 2 10 540 ZF-Mg0.057% 0.28 12.57 0.04 NO 5 IF steel variant 1 10 500 ZF-Mg0.100% 0.31 8.18 0.10 YES 6 IF steel variant 2 10 500 ZF-Mg0.100% 0.29 11.17 0.11 YES 7 IF steel variant 1 15 560 ZF-Mg0.100% 0.30 11.13 0.12 YES 8 IF steel variant 2 15 560 ZF-Mg0.100% 0.29 13.35 0.10 YES 9 IF steel variant 1 20 600 ZF-Mg0.100% 0.30 14.23 0.13 YES 10 IF steel variant 2 20 600 ZF-Mg0.100% 0.26 14.08 0.12 YES 11 IF steel variant 1 15 580 ZF-Mg0.189% 0.47 9.65 0.19 YES 12 IF steel variant 2 15 580 ZF-Mg0.189% 0.42 13.07 0.17 YES 13 IF steel variant 1 15 620 ZF-Mg0.189% 0.44 11.98 0.18 YES 14 IF steel variant 2 30 620 ZF-Mg0.189% 0.48 16.73 0.17 NO 15 IF steel variant 1 15 650 ZF-Mg0.189% 0.47 13.78 0.17 YES 16 IF steel variant 2 30 650 ZF-Mg0.189% 0.43 16.93 0.17 NO 17 IF steel variant 1 15 660 ZF-Mg0.449% 0.30 14.61 0.40 YES 18 IF steel variant 2 15 660 ZF-Mg0.449% 0.21 10.95 0.39 YES 19 IF steel variant 1 20 680 ZF-Mg0.449% 0.23 13.20 0.38 YES 20 IF steel variant 2 20 680 ZF-Mg0.449% 0.22 12.25 0.39 YES 21 IF steel variant 1 30 720 ZF-Mg0.449% 0.32 21.33 0.48 NO 22 IF steel variant 2 15 720 ZF-Mg0.449% 0.24 15.00 0.38 YES

TABLE 4 Alloy Improved Al_(bath) Mg_(bath) T_(galvannealing) formation in anticorrosion Sample [% by wt.] [% by wt.] [° C.] ζ coating characteristics 1 0.129 — 540 + Yes Reference 3 0.144 0.057 540 + Incomplete No 5 0.125 0.100 500 + Incomplete Yes 7 0.125 0.100 560 − Yes Yes 9 0.125 0.100 600 − Yes Yes 11 0.140 0.189 580 − Yes Yes 13 0.140 0.189 620 − Yes Yes 15 0.140 0.189 650 − Yes Yes 17 0.136 0.449 660 − Yes Yes 19 0.136 0.449 680 − Yes Yes 21 0.136 0.449 720 − Overalloyed No Sample material: IF steel variant 1

TABLE 5 Alloy Improved Al_(bath) Mg_(bath) T_(galvannealing) formation in anticorrosion Sample [% by wt.] [% by wt.] [° C.] ζ coating characteristics 2 0.129 — 540 + Yes Reference 4 0.144 0.057 540 + Yes No 6 0.125 0.100 500 + Yes Yes 8 0.125 0.100 560 − Yes Yes 10 0.125 0.100 600 − Yes Yes 12 0.140 0.189 580 − Yes Yes 14 0.140 0.189 620 − Overalloyed No 16 0.140 0.189 650 − Overalloyed No 18 0.136 0.449 660 − Yes Yes 20 0.136 0.449 680 − Yes Yes 22 0.136 0.449 720 − Yes Yes Sample material: IF steel variant 2

TABLE 6 C Si Mn P S Al N Cu Cr Ni No Ti Nb B Sn Soft IF min 0.001 0.01 0.05 0 0 0.005 0 0 0 0 0 0.03 0 0 0 steels max 0.006 0.12 0.15 0.019 0.018 0.06 0.006 0.11 0.1 0.1 0.02 0.11 0.02 0.0008 0.03 Higher- min 0 0.04 0.15 0.025 0 0.02 0 0 0 0 0 0.03 0 0 0 strength max 0.004 0.14 0.8 0.075 0.012 0.045 0.004 0.08 0.06 0.02 0.02 0.065 0.035 0.0013 0.02 IF steels Bake- min 0 0 0.15 0.012 0 0.015 0 0 0 0 0 0 0 0 0 hardening max 0.08 0.17 0.45 0.051 0.012 0.075 0.004 0.09 0.06 0.06 0.02 0.017 0.012 0.0035 0.015 steels Micro- min 0.055 0.03 0.1 0 0 0.02 0 — — — — 0 0.0005 0 0 alloyed max 0.18 0.48 1.6 0.025 0.012 0.07 0.0085 — — — — 0.033 0.055 0.0005 0.03 steels Multi- min 0.02 0 1.2 0 0 0.02 0 0 0 0 0 0 0 0 0 phase max 0.165 1.6 2.65 0.02 0.003 1.55 0.008 0.12 0.75 0.1 0.28 0.13 0.043 0.002 0.03 steels 

1.-12. (canceled)
 13. A process for producing a galvannealed zinc-magnesium hot dip coating on a steel substrate, the process comprising: providing a steel substrate; hot dip coating the steel substrate in an Fe-saturated melt bath heated to a bath temperature of 350° C.-650° C., wherein the Fe-saturated melt bath comprises 0.1%-0.16% by weight Al, 0.1%-0.6% by weight Mg, zinc, and unavoidable impurities; and performing a galvannealing treatment in which the steel substrate that has been hot dip-coated is kept at a galvannealing temperature of 450° C.-800° C. over a galvannealing time of 10-25 seconds to produce a galvannealed zinc-magnesium hot dip coating on the steel substrate, wherein the galvannealed zinc-magnesium hot dip coating comprises zinc, unavoidable impurities, and 0.10%-0.5% by weight Al, 5.0%-15.0% by weight Fe, 0.10%-0.8% by weight Mg.
 14. The process of claim 13 wherein the Fe-saturated melt bath further comprises at least one of <2% by weight Si, <0.1% by weight Pb, <0.2% by weight Ti, <1% by weight Ni, <1% by weight Cu, <0.3% by weight Co, <0.0001% by weight Mn, <0.2% by weight Cr, <0.5% by weight Sr, <0.1% by weight B, <0.1% by weight Bi, <0.1% by weight Cd, <0.2% by weight REM, or <0.5% by weight Sn, and the galvannealed zinc-magnesium hot dip coating further comprises at least one of <2% by weight Si, <0.1% by weight Pb, <0.2% by weight Ti, <1% by weight Ni, <1% by weight Cu, <0.3% by weight Co, <0.0001% by weight Mn, <0.2% by weight Cr, <0.5% by weight Sr, <0.1% by weight B, <0.1% by weight Bi, <0.1% by weight Cd, <0.2% by weight REM, or <0.01% by weight Sn.
 15. The process of claim 13 wherein the Fe-saturated melt bath comprises 0.1%-0.15% by weight Al.
 16. The process of claim 13 wherein the Fe-saturated melt bath comprises 0.01%-0.5% by weight Fe.
 17. The process of claim 13 wherein the Fe-saturated melt bath comprises 0.1%-0.4% by weight Mg.
 18. The process of claim 13 wherein the bath temperature of the Fe-saturated melt bath is 430° C.-490° C.
 19. The process of claim 13 wherein the galvannealing temperature at which the steel substrate is kept during the galvannealing treatment is 540° C.-720° C.
 20. The process of claim 13 wherein the galvannealing time is 12-24 seconds.
 21. The process of claim 13 wherein the steel substrate is a flat steel product.
 22. The process of claim 13 wherein the steel substrate comprises an IF steel.
 23. A flat steel product having a galvannealed zinc-magnesium hot dip coating that comprises: zinc; unavoidable impurities; 0.10%-0.5% by weight Al; 5.0-15.0% by weight Fe; and 0.10-0.8% by weight Mg.
 24. The flat steel product of claim 23 further comprising at least one of: <2% by weight Si; <0.1% by weight Pb; <0.2% by weight Ti; <1% by weight Ni; <1% by weight Cu; <0.3% by weight Co; <0.0001% by weight Mn; <0.2% by weight Cr; <0.5% by weight Sr; <0.1% by weight B; <0.1% by weight Bi; <0.1% by weight Cd; <0.2% by weight REM; or <0.01% by weight Sn.
 25. The flat steel product of claim 23 comprising an IF steel.
 26. The flat steel product of claim 23 wherein the galvannealed zinc-magnesium hot dip coating is free of zeta phase. 